This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-230795, filed July 31, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube of a shadow-mask type.
2. Description of the Related Art
In a color cathode ray tube that is presently in wide use, a mask body of a shadow mask undergoes thermal expansion as electron beams run against it. This thermal expansion (doming) of the mask body causes color drift. In order to reduce the thermal expansion, the mask body is formed of a material with a coefficient of thermal expansion lower than that of the material of the mask frame, in many cases.
The mask body and the mask frame are fixed to each other by two different welding methods. One is based on a system (MOFA: mask outside frame assembly) in which a skirt portion of the mask body is welded to the outer wall of the mask frame. The other is based on a system (MIFA: mask inside frame assembly) in which the skirt portion of the mask body is welded to the inner wall of the mask frame. The MIFA system currently prevails, since it can lessen partial doming during irradiation with electron beams and easily restrain howling that is caused by vibration from a speaker when it is incorporated in a TV set. The mask body is provided with tongue portions that are fixed to the mask frame. The tongue portions are formed by cutting slit-shaped notches in those regions of the skirt portion of the mask body which are located close to fixing portions to be fixed to the mask frame, thereby isolating the fixing portions from the continuous skirt portion. These tongue portions serve partially to prevent deformation of the mask body.
In general, on the other hand, the skirt portion of the mask body is formed by press forming or the like. The press forming involves a so-called spring back phenomenon such that an open end of the skirt portion spreads out. This spring-back phenomenon frequently occurs when the mask body is formed by press-forming a low-expansion material such as invar. If the skirt portion is slit to form welding tongue portions, the spread of the open end is wide in other portions than the welding tongue portions. In some cases, therefore, the other portions may bulge outwards from the welding tongue portions.
In welding the mask body to the mask frame in this state, excessive stress may act on the welding tongue portions, thereby deforming the tongue portions or shifting the welding positions.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube in which a mask body and a mask frame can be prevented from being deformed or dislocated as they are fixed to each other.
According to an aspect of the present invention, there is provided a color cathode ray tube comprising: an envelope including a rectangular panel having a major axis and a minor axis perpendicular to each other; a phosphor screen located on the inner surface of the panel; an electron gun assembly which emits electron beams toward the phosphor screen; and a color sorting mechanism opposed to the phosphor screen, the color sorting mechanism including a mask body, having a rectangular effective portion with a large number of electron beam holes and a skirt portion bent from the periphery of the effective portion and extending along a tube axis perpendicular to the major and minor axes, and a mask frame having an inner wall fixedly fitted with the skirt portion of the mask body, the skirt portion including cut portions, the width of the cut portions along the minor or major axis ranging from 0.10xc3x97L to 0.45xc3x97L, where L is the distance from an end of the minor or major axis to a diagonal end, and the height of the cut portions along the tube axis being equal to 5% to 40% of the height of the skirt portion along the tube axis in a region near the minor or major axis.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.